Secret radio communication



July 19, 1949.

Filed Jan. 22, 1943 S-Sheets-Sheet l H M 32 JLJLJL MASTER PUOLSER PUldgER OSCILLATOR GATF GATE Nqrrpw beam unfenna I F|4 7|5 Broad angle radiah'ng Qnfennq AMPLIFIER MODULATOR (TUNED RECE'VER SHARP) u 24 23 SHOT FIGI.

AMPL!FIER NQlbE "MODULATOR SPURIOUS V (TUNED SIGNAL BROAD) MASTER OSCILLATOR 27A? Inferference beam N ,4 f

Signal beam 2:8 FIG lnferference beam 33 Signal pulse 33 I {A E F 5Q H confinuous B Y lmerference FIG. 3.

INVENTOR ATTORNEY July 19, 1949. R. H. VARIAN 2,435,337

SECRET RADIO COMMUNICATION Filed Jan. 22, 1943 5 Sheets-Sheet 2 20 F IINTERFERENCE I TRANSMITTER i ZB lei 2s A SIGNAL TRANSMITTER FIG, 2A, I INTERFERENCE TRANSMITTER INVENTOR RUSSELL H. VARIAN ATTORNEY y 1949- R. H. VARIAN I 2,476,337

' SECRET RADIO COMMUNICATION Filed Jan. 22, 1943 5 Sheets--Shet 4- Polarized Unpola rized Polarized Rqdiclfion Rqdioh'on Recepfivify SYNCHRONIZED 73 68, I R SYNCHRONIZED MOTOR DRIVE 72 7' x MOTOR DRIVE 73 72 56 GENUINE SIGNAL L TRANSMITTER RECE'VER 74 20 .NTERFERENCE 53.5%; L A..-

TRANSMITTER OR GENUINE SIGNAL TRANSMITTER FREQUENCY CHANGER 71 v 7 RECEIVER 9| l SYNCHRONIZED 54 DRIVE f55A 83 INTERFERENCE PROGRAM "A" 5|GNAL FREQUENCY 20 REJECTER H NGER (POLARIZATI N CHANGER l POLAR'ZAT'ON INTERFERENCE CHANGER TRANSMITTER 56B 5% svNocRl-fcgmzco K FIG 8 SEPARATE PROGRAM "A" 2 SYNCHRONIZED SEPARATE DRIVE SSM PROGRAM "8 558 PROGRAM ":3"

INVENTOR ORNEY July 19, 1949. R. H. VARIAN SECRET RADIO COMMUNICATION a 5 Sheets- Sheet 5 Filed Jan. 22,1943

muPEIm um Im INVENTOR RUS SELL H. VARIAN fl fl, ,7

ATTORNEY mama mMCLZIm mm Im mop/vi 68 $532 mobjaaoz F'atenteci July 19 1949 UNITED STATES SECRET RADIO COMMUNICATION Russell H. Varian,

Wantagh, N. Y., assignor' to The Sperry Corporation, a corporation of Delaware Claims.

My invention relates to systems of communication, apparatus therefor and methods of communication, and concerns particularly systems of radio communication.

An object of my invention is to provide secrecy in communication, with methods and apparatus for enabling only authorized listeners to receive either telephone or telegraphic communications.

A further object is to provide a secret system in which authorized listeners may receive directly and without loss of time for decoding.

In radio communication, the reception of signals by unauthorized or undesired listeners may be made difilcult by transmitting the communications in as narrow a band of radio frequencies as possible in order to make it diffi-cult for the undesired listeners to find the proper band on which to listen. In a case of radio telephone communication, for example, the band width may be reduced to 3000 cycles or less, although excessive compression of the band interferes with the quality of the reception and increases the difiiculty encountered by authorized listeners in obtaining intelligible signals.

Furthermore, the difiiculty of undesired listeners in receiving the signals may be increased by causing the signal level to rise relatively little above the noise level at the point of reception. In this case, however, the noise level is relatively fixed at various points of reception and unauthorized listeners close to the point of transmission will receive signals well above the noise level of their receivers. Thus merely holding the signal level down to a point slightly above the noise level at the desired ineffective in eliminating reception by undesired listeners at other points which are closer or more advantageously located, so that at such points the signal level is substantially higher than the noise level.

Compression of the signal to a narrow frequency band and holding the signal level down to the minimum necessary for reception by authorized listeners tend to make the discovery of signals by undesired listeners more difiicult but do not prevent satisfactory reception of such signals after they have been found and may not prevent the discovery of such signals by skillful operators.

7 It is accordingly an object of my invention to provide methods and apparatus for both preventing the discovery of signals and preventing the reception of signals by'undesired listeners regardless of the skill ofv the receiving operators point of reception would be or the perfection of the receiving equipment with which they are provided.

In carrying out my invention in its preferred form, in order to make it equally difiicult for an undesired listener to find the signal in either an advantageous or disadvantageous location, I provide means for transmitting interference or noise together with the genuine signal so that the ratio of signal level to noise level will be substantially the same throughout the area within which undesired listeners may attempt to pick up signals. -In order to increase further the difiiculty of undesired listeners in finding the signal, I utilize a system of transmission in which the transmission of genuine signals is relatively narrow or sharp and the transmission of interference, noise or spurious signals is relatively broad. I may resort to difference in breadth of transmission of the genuine signals and the interference in one or more of several different respects, such as with respect to space, frequency or time, for example. Thus I may use a highly directive radiator for transmitting the genuine signals over a very narrow angle of azimuth and simultaneously use a broader angle radiator for transmitting spurious signals or interference over a very wide angle so that at very nearly every point except at points in the line of the authorized receiver, the interference level will be as high as or considerably higher than the genuine signal level.

With respect to frequency, I may transmit the genuine signal from a very sharply tuned transmitting system, or I may confine the genuine signal to a very narrow band of frequencies; whereas the interference signals are transmitted over a very wide band, so that an unauthorized listener resorting to a wide band receiver to find the signal would encounter high noise levels and will hear only interference at every setting of his receiver.

For instance, if a channel 3,000 cycles wide is reserved for the signal, and the noise level is such that a wider band receiver cannot be used, and the signal is anywhere between 3 10 and 6 10 cycles, (5-10 cm. wave length) the unauthorized listener would have to search through 10 reception bands. If the searcher spent only one-tenth second per band, it would take days to make the search.

Of course it would take a great deal of power to spread noise over such a band width. Hence, in practice, an operator would not go to such extremes, but would change the transmission frequency at frequent intervals according to a .iment of 3 so that the search time would longer than the time spent on one pro-arranged plan be much frequency.

Furthermore, with respect to distribution of signals in time, I may utilize a pulse transmitter in combination with a receiver which is sensitive only during the intervals corresponding .to the pulses of the transmitter for the transmission of genuine signals. Simultaneously, I may transmit over much interference either continuously or longer pulses 'vention.

Fig. 1A is a schematic diagram of an antenna array which may be used for the apparatus of Fig. 1.

Fig. 2 is a radiation 'pattern of a transmitter which may be used in connection with a modified embodiment of my invention.

Fig. 2A is a schematicdiagramof an antenna array which may be used to obtain the pattern of Fig. 2.

Fig. 3 'is a graph explanatory'o'f the principle of operation of my invention in connection with the confinement of genuine signals to narrow time bands and the transmission of interference continuously or over wide-time bands.

Fig. 4'is a schematic diagram of -a transmitter forming another embodiment-of my invention.

Fig. 5 is a schematic diagram of a receiver adapted for use with the apparatus "of Fig. 4 and forming therewith "a system-constituting anembodiment of 'my invention.

Fig. 5A is a'block diagram of a radio communication system forming another embodiment of my invention.

Fig. 6 is a block diagram of a modification of "the arrangement of Fig. 5.

Fig. '7 is a graph explaining the principle of operation of theapparatusof Fig; 6;-and

Fig.8 isa block diagram of'aradio'comrnunlcations system'formin still another embodiment of my invention.

Like reference characters are utilized throughout the drawings to designate like parts.

In Fig. 1,'I have represented schematically a radio communication system forming one embodmy invention and in which the'apparatus is assumed to be of the type utilizinga'mo'du- 'lated oscillator for transmitting a voice, telephone or modulated continuous-Wave code signal. However, my invention is not limited'there- 'to and the invention is explained'in connection with such conventional elements of modern radio transmitting systems merely for'the sake of illustration.

For transmission of the genuine signal-to a 'desired or authorized listener .Iprovi'de radio transmitting apparatus'including means for-transmission of the genuine signal-such means being represented by the group ofrectangles ll, and'ITp'ro vide receiving means-represented by a group of rectangles l2. In case a modulated radio'frelistener, the'rece'iver'la'is tuned to this same freso that a receiver which is not designed for reception during the same pulse accompanying but uttering misleadin quency type of transmission is employed the gen uine signal transmission means ll comprises a master oscillator 13, a modulator M for modulating the output wave of the oscillator 53, in accord-- ance with the intelligence to be transmitted, and if desired, an amplifier l5 for increasing the power of the modulated signal before application thereof to a radiator or antenna 16 represented for convenience as being of the dipole type. For actuating the modulator l4, suitable means H is provided for impressing an audio signal on the apparatus. This may take the form of a keyed audio frequency oscillator, a microphone spoken into 'byfthe transmitting operator or the like.

The receiving means includes a conventional receiver l8 suitable'for reception of signals of the type transmitted by the apparatus I l and having suitable means such as antenna or loop is for picking up the radiation from the transmitting means I I. It will be understood that the receiver 18 is tuned to the principal frequency or carrier wave of the transmitting means H and preferably is tuned sharply =0r has only sumcient band spread to receive the sig-nals-efficiently from the means H.

For the purpose-of making it difficult for unauthorized listeners :to findithe genuine signal :or to receive the signal, if =perchance the unauthorized listener should make .an adjustment ..of his-receiver suitable for best reception of the genuine signal, I .provide supplementary transmission bands or other means, which in the case of the apparatusof Fig. 1 constitutes a. separate transmitter 20. The transmitter 20 may be of the same general type as the transmitter ll. In this case, it is also a transmitter ofthe-type comprising a master oscillator 2|, a modulator 22, an amplifier 23 and an audio input device .24. The audio input device 24 may be :a source of noise, spurious signals or other interference. For example, it may'consistof adevice suchas a resistor -or a saturated-diode vacuum tube together with be a keyed or interrupted oscillator having the same 'wave form .and'keying frequency, but-arranged to produce "no intelligible signal by its "keyed codejor to produce only "spurious signals with faulty information. Likewise, if 'the enuine audio signal source H is a-telephone-microphone intended to be spoken into, the interference source 24 may bea phonograph "or transcription device "containing .a :recording of the same voice speaking into the genuine audio signal-source if! though plausible soundinginformation. V

The transmitter -20 including all of its elements,

particularly the oscillator 2 I, amplifier 23 and the radiator 25, .iszarrang-ed to transmitover a broad range whereas the transmitter H including the correspondingpertinent elements, is arranged to transmit over a narrowrangein order thatthe desiredlistener knowing the range over which to listen, will be -able to .find the signal whereas the undesiredtlist'ener .will .have great Jdifliculty in finding the. signal. Forexample, if the apparatus II is arranged'to'transmitsignals over a very narrow frequency range known to the desired quency, the desired listenerwill receive the,

genuine signals without difiiculty whereas the unauthorized or undesired listener attempting to find this signal will find that the genuine signal is so overlaid with diifuse interference from the apparatus 20 that such undesired listener will be unable to find the proper adjustment of the receiver for picking up the genuine signals. Thus, if the undesired listener utilizes a receiver which tunes very sharply it will be virtually impossible for him to find the signal. On the other hand, if heuses a broad tuning receiver so much interference will be received from the apparatus 20 that there will be no indication of the presence of thegenuinesignal I! in they reception, for any adjustment of the undesired listener's apparatus.

Although a, very convenient and satisfactory way of masking the genuine signals is to transmit and receive these signals on a narrow frequency range and to overlay the genuine signal with an interference signal spread over a very wide frequency range, my invention is not limited to this particular form of masking the genuine signals. Nor is my invention limited to this mannerof using a narrow range transmission for the genuine signals and broad range transmission for the maskin interference. For example, in place of broad and narrow frequency range, I may also make use of selective space radiation, or the transmission of the genuine signals over a small angle of azimuth or small range in space, with transmission of the interference or spurious signals over a large angle of azimuth or wide range in space. For example, the radiator I6, and to the extent necessary, the other apparatus constituting the transmitter l I, may be arranged to produce a highly directive radiation represented by the signal pattern 26 whereas the radiator l6, and to the extent necessary, the other apparatus forming the transmitter 20, is arranged to produce a signal either having no directive properties or which has a much wider angle of radiation represented by the radiation pattern 27.

Apparatus for producing more or less directive radiation such as represented by the patterns 26 and 21 is known to those skilled in the art and does not constitute part of my present invention and need not'therefore be described in detail.

More or less directive radiation, such as represented by the patterns 21 and 26, may be obtained in any suitable manner according to the type of apparatus and wave-length employed. In the case of microwaves the angles of radiation may be controlled by the shape or size of the reflector or horn employed. In the case of longer wavelengths suitable antenna arrays may be employed. For example, the broad angle pattern 2'! may be produced by a pair of vertical dipoles spaced onequarter wave-length and fed with current having 90 difierence in phase, as shown at page 419 of Ultra High Frequency Techniques by Brainerd, Koehler, Reich and Woodrufi, published in 1942 by D. Van Nostrand Co., Inc. The narrow angle pattern 26 may be produced by larger numbers of vertical dipoles suitably spaced and with suitable phase relationships of current. For example, as shown at page 420 of the aforesaid Ultra High Frequency Techniques, with an array of sixteen dipoles, they may be spaced 5% wavelengths apart with 157.5 phase difference between currents in adjacent dipoles. An illustrative arrangement of dipoles constituting antenna arrays l6 and 25 is shown schematically in Fig. 1A.

In case a system Of broad and narrow angle radiation-is employed the pick-up means .19 of the receiver l2 will of course be located along the line of maximum radiation of the transmitter l I, substantially the center line of the radiation pattern 26. As indicated in Fig. l, the radius (indicating signal strength) of the radiation pattern. 26 along its center of symmetry 28, is considerably greater than that of the radiation pattern 21 indicating that a receiver placed along this line or within a relatively narrow angle along either side thereof, will receive stronger signals from the; genuine signal transmitter ll than from the interference transmitter 20. However, anunauthorized or undesired listener who does not know where to place his receiver is most likely to be located outside the narrow angle represented by the radiation pattern 26. For example, if his receiver is along the line 29 it will be apparent that, since at this point the radiation or signal strength of the pattern 21 is considerably greater than that of the pattern 26, the interfering or spurious signals will be far stronger than the genuine signals and it will be impossible for such an undesired listener to make any adjustment. of his receiver which will enable him to receive the genuine signal.

Although for the sake of convenience, I have separately described the arrangements for masking genuine signals by broad range interference signals with respect to frequency range and with respect to space range or angle of radiation, and either method may be used without the other, it will be understood that both methods may be and advantageously are used simultaneously.

Furthermore, I may also make use of a narrow distribution of genuine signal with respect to time and a broad distribution of the interference with respect to time, separately or together with one or both of the other means of masking genuine signals which have already been described. For example, the transmitter it may be provided with a pulser or gate 36 separately represented for convenience b the rectangle 30 and which is arranged to give the output of the transmitter H a characteristic represented by the symbolic pulse or square wave graph 38. In connection with such pulsing systems which do not in themselves constitute my present invention, the output signal is transmitted during short intervals, such intervals having a duration of 10 micro seconds, for example, which transmission intervals or pulses are repeated at much longer intervals. They may occur at rates of 2000 pulses per second, for example. In this case the receiving means 12 likewise includes a corresponding device which may effectively be referred to as a pulser or gate 32 which makes the receiver sensitive only during the transmission intervals or pulses of the transmitter H.

The gate 32 is necessarily synchronized by suit able means, not shown, with the pulse rate of the transmitter l i but such means do not constitute a part of my present invention and are known to those skilled in the art.

The interference transmitter 20 may, if desired, be provided with a pulser similar to pulser 30 but having either a different pulse rate or producing much longer pulses. Consequentiy, any receiver not knowing the pulse length and pulse rate on which to receive will have the reception blanketed by strong interference because the receiving apparatus will be sensitive during such long intervals of time while the transmitter interference is being received, that the interference will be much stronger in proportion than the genuine signal even though the arages most 'certainly' beblankedout by interferencei v Thisis illustrated in-Fi g-a 3i Asshow-n in Fign. 3," the genuinesignal oceurs: in pulse's 33: which are of a short duration compared with tlre inter vals-Bfl between pulses. If the interferenoe trans mitter 20 operates co tinuousl or transmits I in 1 long pu1ses,='itwill be seerr athat='thetota:1energy. transmitted by the interference transmitter will be-far greater thanthe energy in the genuin'e'. signal transmitted during the pulsesfit.- Ax con'itin-uously sensitive receiver will be affected by the total energy! As shown in FigB, the areaABGD' represents -'-the "energy present 'when 'onlystheaim I terference is =bei'ng transmitted} and? the area which is verymuch smaller, represents I the energy of the genuine signals If the inteni-i .1

sityof-thegenuine signal AE 'is made'sslightly greater than theinten's ty AB oi /the interference-'- signal; it will 'be'obser've'd that the excess energy during the pulse inter-Val repr.esen ted'.i'by -the pulse area BEFl-lisvery considerably :smallerthan the energyrepresented by the" area DCHG, cor-responding to" th'e- -energy of the: interference signal-n 7 -Thus, it is apparent 'that I -have'described a secret communication system with means; for ra diating a genuine signal n arrowh 'restrictedin a discriminating parameter; :means for radiating; a masking: interference:signal much' lless narrowly restricted by the same discriminating parameter,

and means forireceiving a signal having 1substantially identical restrictions onvreception in thesame discriminating parameter" as the genuine signal radiating means. Various parameters may be employed, such as frequency, space an- 'gle, and time, as already described, but my=inven- 4m tion is not limitedto theseparameters.

In connection WithFig. i, Ivhave described .anarrangement for blanking genuine signals transmitted over a narrow angle of radiation by-:--intipi-dipole -'arrays magi be laeeitfrm ctnre m gsi. 25A} [Bi and 25B;:maling $m'all angles to -bath other s reriresented in Figiizki In connection -with the maskingziot genuineisign nals' byi rinterference ignalsspread oven'a broad*-- range of frequencies have-:heretofore implied-' that such interference signals are spread-1 through-"== outthe broad-range in: question and-'ithat the gen uine signals are confined-i to: a fixed: narrow range': onf-requenciess: l-Iowevr; my invention is not lime ited-tlrereta Ifdesireda thei interferenee signalmay be conce'ntrated to a: greater or l'ess extent-111': several ditferent parts of the br'oad ranges andi furthermore, th e genuine I signal: may-be trans-" fer-red -from one freqiiency to anotheifiaccordin ito aj-predetermined -pattern-ior program -iin orders toinereasethe difficulty eneountered by an um authorized disteneri: For exampleeastillustrated in Figsi 4 and 5, the tra'nsmitting apparatus masci include a plurality ot transinission b'axids for in terference' o'r spurious si'gnals:and atransmissiom band for genuine si'gnals witli m'eariS f ori'inte' changing the bands' on' which the genuineiandiin ter-ieren'ce signalsare:- tr'ansm-ittedl Althoug-bl separate radiators may be:employedforidifrer'ent transmissionm'ands my inventicn is inotilimited thereto and; as 11 illustrated: in connection with i FigsI-4- and -5,' the =transmittingapparatns may be provided-"withditierent transmissiorr channels allof which useth'e same' radiator a The transmitte rep resented sohematicallyiiim Fig:4,- corrlprise'sa master oscillator l-3=,'- eai highi frequency modulator 3 sy aradi'o:requeney anemi fier I55 and asuitabl an'tenna orlradiator -lfi Fonthe purpose' of causing trahsmission on sey era "different frequencies iro'm 'the radiator lfig thet highirequency' modulator: 36 is- 'modulat'ed at at pluralityof 'difirent ir e-queneies:nignerztnan'tne I audio frequency signals to- :betransmitted but! referably lower than the 'frequency ofahe master oscillator 3 whichdetermines the -princi-palcaxi-'=-= ri'er frequency or: theradiationl from the aantehna'i I6. To this end-l provide a plurality o'f oscillators I have a designated 'supplementaryi oscnterference signals transmitted over-a broad an- 5l 1 az g and 395463611 havif ga differ nt;freg-g gle ofradiation. My invention inthis: respect, however, is not limited to the use .ot-radiators:

having radiation patterns of a specific-formnllustrated at 26 and 2'1" in Fig. 1. Forexample, .in

place of a broad angle interference radiation having the pattern Eiof Fig. 1, I may utilize'an in terierence beam which isisplitinto two re1atively narrow parts 27A and Z'iK-together with a genuine signal beam having a radiation pattern 26 directed along a line midway between the spiitpatterns 21A and 2l-B of-theinterferenca signal. The arrangement'ofi Fig. Z-has-theadvantage that along the line 28011 which the authorizedreoeiver is placed-noise or-zinterierence' reception is very nearly nil whereas-at a few de+ ov grees either side of theline 28,- interference reception becomes very strong and the genuine: signal reception falls offvery rapidly; Thereis; of course, no need for the-interference beam on either side of the loops ETA-or 2'iB,'-for example,

along the line 35, if the transmitting apparatus H is correctly designed, since thereWiI-Lbeno genuine signal beam at this-angle. Care shouid; of course, beta-ken in'the design-of thoappara tus ll andthe radiators IE to insure the ab-- quency' from the other supplementary oscillators Corresponding-"to the supplementary oscillators 3'! 38 and 3 9; arefconventional audio i frequencymodulators 405' Hg and 42 Audiorfrequency'signa'ls' from sources reprles'entetl by i ectanglee 4 and 45' areimpnesse'd on' the modulatorsadngal hi: and 42 each modu1ator'having audioisignals froma only a single sourceiimpressedstliereons at: anyi given: instant. -The modnlatedifrequencyzoutputsi resulting 'from the -:combinations of the unitsifl and .4 0; the :units s stand d I andithe units 39 an -i 452;. r impressednofi .the high -fre'quency:modulator 36 soa-that the output of *the master roscil latorfl 3 is doublyv modul'atedz and: a large-number: ofi'side-bamls istransmitted'tthroug-h the amplifier lia 'toathe antenna l6and radiated-therefrom:

My inventions is not 'limited 'to theuserof1 any? specific frequencies, frequencyiratioaor frequency; diffie'rence Nevertheless'y' for the sa'k'e of conve'nience Yin: :explanation I and bye. wayi 'of rillustra e tion; but not by; way; of 1 limitation various arbi trarilyiselectedfrequencies-have been :designated::- in various parts: ofthe apparatus to diStihguish one apart :from lanother-and'rwillbe referred toin the subsequentidiseussionzY For example; let" it be: assumed-that the: masteri oseillaton produces a; primary; ..ca'rrier a at v 30;:me'gacycles ands that the supplementary oscillatorsfiil \38-;--and= 39'= produce supplementary onsubsidiaryr carriers at -5meg-aany suiteblemanllfih q a p a three mul cycles, iloimegacycles and lsemegacycles res ec 'tively. Then when the audio sources 43, 44, and

frequency modulator 36, producing three upper side bands and three lower side bands at 45, 40, 35, 25, and 15 megacycles. If it is assumed that the frequency components of the audio frequency sources do not extend beyond 5 kilocycles, the outermost side bands produced by the modulator are 5,005 and 4,995 kilocycles. Similarly, the outermost side bands of the modulators 4| and 42 are 10,005 kilocycles, 9,995 kilocycles, 15,005 kilocycles. and 14,995 kilocycles. If these side bands are impressed on the high frequency modulator, the radiated wave from the antenna [6 will include additional side bands as follows: There will be side bands at 45,005 kilocycles, 44,995 kc., 15,005 kc., and 14,995 kilocycles, resulting from the modulation produced by the audio source connected to the modulator 42 combined with the subsidiary modulation of the 15-mc. supplementary oscillator 39. Additional side bands resulting from audio sources acting through the 10 megacycle supplementary oscillator 37 and the 7 cycle supplementary oscillator 38, will produce additional side bands 40,005, 39,995, 35,005, 34,995, 25,005, 24,995, 20,005 and 19,995 kilocycles. Thus it will appear that when only three difierentaudio frequency modulators 40, 4| and 42 sponding supplementary oscillators 31, 38 and 39 are used, a large number of different side bands are produced in the radio frequency radiation.- In practice, I may employ many more supplementary oscillators and audio frequency sources placing :1.

interference or spurious audio frequency information on all of the audio sources except one on which the genuine signal is placed. In order to separate the genuine information from the spurious information or interference, the authorized listener is provided with a receiver which selects either the upper or lower side bands corresponding to the genuine audio frequency information.

As illustrated in Fig. 5, the receiver may be of the superheterodyne type although not necessarily so. If the receiver is of type, it may comprise a conventional local oscillator 46, a mixer 41, a radio frequency amplifier 48, a receiving antenna or loop 49, an intermediate-frequency amplifier 50, a second detector 5| and head phones, or the like, 52. Assuming that the genuine audio frequency information is placed on the audio frequency modulator 42 corresponding to the supplementary oscillator 39, upper side bands in the radiated carrier will be produced between 45,005 and 44,995 kilocycles and lower side bands will likewise be produced between 15,005 kilocycles and 14,995 kilocycles. The receiver may be designed to select one or the other of these groups of side bands. For example, if the receiver has an intermediate frequency amplifier designed for 1500 kilocycles and if the upper side bands of the radiated wave are to be selected, the local oscillator 46 may be adjusted to oscillate at 43,500 kilocycles. The local oscillator 46 will then beat with the intermediate side band at 45,000 kilocycles (which is produced by the 15-megacycle supplementary oscillator 39 modulating the oscillator 34). since this difference between 45,000 and 43,500 kilocycles is 1500 kilocycles,.the beat frequency will be 1500 kilocycles andthe signal will be picked up in the seconddetector 5| and the head phones 52, being modulated at an audio frequency corresponding to the original input audio frequency as a result of the spread of the the superheterodyne 5 megaand correradiated carrier between cycles.

If only three different supplementary oscillators 31, 38 and 39 are provided, the local oscillator 36 is provided also with two other adjustments. For the specific frequencies assumed, these may be 38,000 kilocycles and 33,500 kilocycles. Having been informed on which of the supplementary oscillators the genuine signal is to be'plac'ed, the authorized receiving operator adjusts his receiver accordingly and selects the genuine signals, tuning out the spurious signals or interference.

In order to increase the dificulties of the unauthorized listener a greater number of supplementary oscillators than illustrated may be provided, and furthermore, means are preferably provided for rapidly changing the genuine signal from one set of side bands to another. Corresponding means are provided in the receiver for shifting the signals according to the same program. For example, a change-over switch 53 may be provided in the transmitter of Fig. 4 and a corresponding frequency changer 54 may be provided in the receiver of Fig. 5. The frequency changer 54 may be a conventional device such as a selector switch for changing inductances or capacitances in a tuned circuit of the oscillator 46. The change-over switch 53 and the frequency changer 54 are driven by suitable devices for causing them to follow the'same program. For example, chronometers or synchronizer drivers 55 and 56, respectively, may be provided. The synchronizer driver may be of the type used in radio television utilizing synchronizing pulses but my invention is not limited thereto. Particularly, when it is desired to eliminate any possibility of trouble from jamming of signals, it may be preferable to utilize ships chronometers for driving the change-over switch 53 and the frequency changer 54. Since my system is particularly useful for the rapid secret direct voice communication between ships of a fleet, the changeover switch 53 and the frequency changer 54 may readily be driven by chronometers on the respective ships.

In connection with the apparatus of Figs. 4 and 5, I find'it advantageous not only to limit the genuine signal to one of the audio sources at the source 43 but also to provide different spurious signals on the other audio sources, such as the source 44 and 45.- For example, the audio source 43 may consist of a microphone spoken into by the communications officer on the transmitting ship, and the other audio sources, such as the sources 44 and 45 may consist of phonographs having separate recordings of the same voice but different sentences or words, each recording stating plausible sounding information which, howevenreveals no useful information to an unauthorized listener.

To increase the difficulty of an unauthorized listener in attempting to record reception on a large number of different frequencies corresponding'to different side bands, and later trying to select the information which is genuine, I prefer to operate the linkage 60 between the chronometer 55 and the change-over switch 53, and correspondingly, between the chronometer 56 and the frequency changer 54, in such a manner that the changeover switch 53 and the frequency changer 54 are operated at a high rate of speed so as to clip ofi the words of the intelligence transmitted on any given set of side bands. Consequentlyan unauthorized listener continuously tuning to anyone of the given different frequencies on which the 45,005 and 44,995 kiloericcordingly, the change-oyereswitch 53is pref- -;erab'1y: ofxawsuitable t-ty-pe-.such::- as 311 electronic switch. or fmechanioalswitch adaptedxfor opera- :tion at a: high rate .of speed and producing-rela- '-.t-i-vely-tlittle. frictional :load onther-chr'onometer 55. .My. invention is.-not limited to'the use .of. a specific t-formzof-changeover switch but forthesake-of ilulustration. Ihave schematicallyrepresented a me- -c-hanical formofswitchcomprising threerotating members 51, 458-and 59 ,-..driven through a suitable .li-nkage 60 bytthetc'hronometer 55. The rotating .member- .-51 comprises ta conducting segment :6 l and a non-conducting segment 62, the latter storming a greater part of the periphery of a circle. :mhetang'ular'. length-:of the segment 6| will, of ..course, depend -.-.on:-th'enumber 'of audio, sources providedrandt'herlengthof.timeeach of which is .to -beiconnectedetc-each of; the modulators- 4| and :42. aAlthough-,.ordinar;ily.- more than-three audio sources will lzberemployed, .in the case of :three .audio sourc'es; thev angular length of the segment l willube slightly under 120. Cooperating with zthe rotating 'member: 51--is; a: plurality .of brushes l63,,-54.1and\65;leachconnected toia dirferentone of :thezmodulators Mi -.41 and. There is alsoa --..brush- 56 connected. to the output "of one =of the audio-rsources-,- suchrraslthe. audio source 43. The .conducting. segment 6 I is;extended alongthe axis in such a manner-that itvmakes-continuous con- ,tact with the brush'BB. 1Rlotatingzmembers' 581and 59.lare similarly-constructed and arranged.

From the: drawing-,.- it will be apparent that :as thelinleagerfill rotates theimembers 51, 58 and-59, the brush 66 will "be iconnected "successively through'thesconducting segment 6 I? toithe brushes 163 ,1 642 and 65; Lthus connectingzthe output :of the audimsourcekflzsuccessivelyto the modulators 40, I'tIJand :42. isAtsimilarractionttakes z'place with-'re- -spctrtothe ontputsof'theraudio outputs and 45, iexcepttthat reachqaudios source .:.is: connected to a different zone-20f :theamodulatorssat :any given in- ?stant. acIt isfialso apparentithat asra'rresult :of the rotation of the members 51, 58 and 59 the-audio tfrequency modulator 40:1-is connected successively ct(lithe1audio.scrurceill3; t'll4..and a' similar condition eexists withrespect'sto fthe: other audio frequency monulatorsfl hands". -ronsequently;tthe genuine Faiidio frequency -signals ;:applied to the'uaudio csuurce 113L316 vanons'tantlyl being shifted from one zest 'of side bands to: the iother and. each set 10f side .bands sis successively :carrying signals resulting ifrumnifferent audio-sources; f'T-he constant rapid Tshiftingi'abont: from'oneicarrier to'the other makes :itcimpossible: for antunauthorized'listenerto receive. any; .-intelligible signals. The program 1 acicordingtto whi'chz the. shifting-takes place =may, of .cmltrseibelchangedif rom timeftoitime by changing therrate :of: speed at which the change-over switch :-:53 :an'd:therfrequency-tchanger 54 are driven or by substituting :diff erent..=change-1over 'switch' units swithidifierentprograms. ?:Furthermore; 'my invenition is 'not'limited. torthesuse iof'the relatively simiple rotaryzsuccessionzprogramjust=explained.

.E'It: will ialso :be: .n'otedlthatz if the unauthorized listener; succeeds; in accomplishing 'the' =.prper transposition of bands so that" he obtains inteleligible infonmationyheisistill"in a state of uncertainty because "in theTillustrative -system described 'for simpli'city th'ere are three different programs I which can give intelligible inform-atiom and two" of these give false information. with a' greater number of audio sources "than 12 fillllStIlfltEiJZ-hfl. unauthorized listener would-have .even greater; difliculty.

. Anotherarespect in which :the .range .of-distri- -bution 50f; genuine lsignal radiation may-be .made different:from that-rof the masking interference is. the polarization of'the radiated wave. For ex- ,aample, as, illustratedin-Fig. A,;the transmitter l hand-thereceiver 18 maybe arranged totrans- .mit andreceive waves polarized in apredetercmined fplane, whereas theinterference transmit- -ter 20.-:-is-.-:arranged to transmit-waves polarized inl ardifferent-manner or unpolarlzed. invorder to; increase the difficulty rotreception by unauthorized .1isteners, they-plane of :polarization 51S 5 preferably varied either -by phYSiCa-LTOtdtiOlTiOf .therztransmitting andtreceiVing antennae --or by eelectrical :rotation ofthe' plane.

-.;.1n.-j:he specific arrangement illustrated :in Fig. ,-5A,.-*.the;genuine-signal transmitter l I is provided .-with'a radiator-61 designed totransmit wavesrpo- :larized in a predetermined .planaand thereeciver .l8 ris provided with areceiving antenna T68 ':arranged to *be responsive to'wavestpolarizedin the .same plane as those-transmitted by the antenna 1. The. antennaefi'l -and:58 are arranged Jobs-rotated in unison-by asuitable 'meanssuch .as-esynchronizedrmotor :drives -v and- 56. respecrtiveiy, which-.zmaw be SYIlChIOl'liZBdiiIl zany 'suitvablelmanner as bye-means of ships chronometers at :the t transmitting and receiving stations; :r-eispectively. ":Eor permitting rotation of 151181-1811- eten-nae'fil .-;and-68, suitable connections "to :the transmitter 'l '1' and receiver I Bare provided'which rarerrepresented-.f.or simplicity in thedrawing'zas -=-slip siting:- andl brush. connections 69 and -18! ,relspectiyely (although: it is well :known to those skilled inthelartqthatv othertypes of connections to-a rotating elementareused.

"Eor rincreasing .the :confusion-sencountered by z'anmnauthorized: listener, "the synchronized :drlves :55eand-f56 maywbeaarranged to produce rotation iii-accordance with a;predetermined :progr am with variations't'in' speed or directionpf-irotation at pre- -determinedintervals. Forwmasking the genuine .signalr radiation 'from the transmitter ll: and pire-venting any unauthorized rlistener from at- .tempti-nglto.decipher; the programyin accordance -withtwhichrthei antenna 61 is' rotated; the inter .ferenceltransmitter--20: may be provi'ded with. an antenna system .J- l which; transmits'radiation ex- .-t-.tending-outside :of the plane ofpolarization-rtof rthelradiationl from the lantennafi I. example, xtheantenna'zsystem' 'H--may :consist of atplu- -rality.-.of:pairsof antenna .elements 12 and :13 which/are excited;:incoondinately with respectto neach other .to :produceunpolarized radiation.

.znAlternatively :suitable phase-shifting, ampli- -.tude-.controlling or other selective control means representedgenerally in-Fig; 5A by the. reference mumeralrfll may; be interposed in the. connections between ionergof -the:transmitters "I I or -20 and ;.the correspondingradiation systemifiliand 1 l for .causing::polarizationz or variation' in polarization s'accorldingr to different patterns'from' the antenna Withgthis arrangement;.-an mnauthorizedalistenerattempting to set up are- -.:cei-ver:anditozcoordinate it with the transmission -fromlth'e'genuine signal'transmitter II will be prevented from-:find-ing the genuine signal and Efrem-receiving it' by the interference according to afidifierentpolarization pattern from the'interiference' transmitter20. "For instance, the apparatus' 'l l m'ayincludea 190 phase-shifter on oneantennai This will produce circularpolariza- 7 5 tion which cannot be eliminated by orientation 0f the antenna, and if the direction of rotation of the. polarization is frequently reversed, a circular polarization reeciver cannot eliminate it.

One manner of accomplishing variation of the polarization pattern electrically is illustrated more in detail in Fig. 6. In this case the transmitting apparatus including the genuine signal transmitter II and the interference transmitter 20 is shown as having a common radiator or antenna system 15, and the receiving apparatus i2 is shown as having a corresponding antenna system 16. The antenna system '!5 comprises a polarized antenna 11 represented schematically as being of the dipole type. A second similar polarized antenna 18 is arranged at right angles to the first antenna 11. One of the antenna is connected to the amplifier l so as to be energized thereby, and for energization of the other antenna 18, a second amplifier I5 is provided having a phase shifter 19 interposed between it and the modulator M. In this manner, polarized resultant radiation takes place from the antenna system 15, but the radiation is polarized in accordance with a difierent pattern from plane polarization. For this arrangement there may be a common master oscillator l3.

For making the receiving apparatus i2 responsive to the radiation from the antenna system 15, the receiving antenna system 76 is arranged in a similar manner with a polarized antenna 80 corresponding to the antenna 17 connected through an amplifier 8| to the receiver or detector [8, and a second polarized antenna 82 at right angles to the antenna 80 connected through a second amplifier BI and a phase shifter 83 to the detector or receiver I 8. The phase shifters 19 and 83 are so set that the polarization pattern of the receptivity of the receiver corresponds to that of the radiation of the transmitter. Thus, in the specific case illustrated, a circular or elliptical pattern may be produced according to the degree of phase shifter employed and the relative intensities of radiation from the antennae l1 and 18.

For masking the transmission from the antenna system produced by the genuine signal transmitter H, the interference transmitter 20 may be arranged in a similar manner to the genuine signal transmitter ll having a pair of amplifiers 23 and 23' energizing the antennae T! and 18, respectively, with a phase shifter 84 interposed between the modulator 22 and the amplifier 23. The phase shifter 84 may be so adjusted as to produce resultant radiation polarized in accordance with a different pattern from the resultant radiation produced by the genuine signal transmitter H.

I may also produce synchronized variation in the pattern of polarization of the genuine signal transmission and of the receiving apparatus receptivity by utilizing the phase shifters 19 and 83, which are driven synchronously by suitable devices such as synchronized motor drives 55 and 56. In this case the genuine signal transmission and the receptivity will be in accordance with a polarization pattern which is constantly varying from planar to various degrees of elliptical and circular polarization.

As illustrated in Fig. 7, when the output from the amplifiers l 5 and I5 is in phase, the radiation produced thereby will be polarized with respect to a plane 85, shown in an arbitrarily assumed As the phase shifters 19 and 83 are changed in adjustment, the [polarization pattern will become elliptical, as represented by the curve the phase shifters 19 and relations, the polarization pattern will become circular, as represented by the curve 81 in Fig. 7.

The phase shifter 84 of the interference transmitter 20 may be driven at random or in accordance with a different program from the synchronized drives 55 and 56 by a driving mechanism 88, thus super-imposing on the genuinesignal radiation from the system 75 radiation which is polarized but falls outside the pattern of polarization of the signal transmitted by the transmitter I I and which likewise falls outside the corresponding receptivity pattern of polarization of the receiving apparatus l2.

As already explained, I may cause one of the characteristics of the genuine signal transmission to vary in accordance with a predetermined program, causing the same characteristic of the receiver to vary in accordance with said program in order that the receiver will follow the genuine signal transmission through any masking interference. Also, instead of varying such a characteristic of the genuine signal transmission, I may vary a characteristic of the interference transmission and cause the receiver to follow the variation in characteristic of the interference transmission in accordance with the same program in such a manner as ltO eliminate or reject the interference transmission. An example of a system Where frequency is the variable characteristic and Where both the genuine signal transmission and the interference transmission have the variable characteristic is represented by the embodiment of Figs. 4 and 5. In this case, however, the variation program of the interference transmission must be coordinated with that of the genuine signal transmission owing to the fact that both transmissions have the same characteristic varied, namely, frequency. Thus, the receiving apparatus of Fig. 5 is simultaneously made receptive to the genuine signal transmission and unreceptive or in a condition to reject the various interference transmissions.

The variation program of the interference transmission may be wholly unrelated to the variation program of [the genuine signal transmission if different characteristics are varied in the genuine signal transmission and the interference transmission. Various characteristics have already been mentioned, such as f-requency, space angle, time, and polarization, e. g. Ordinarily, it will not be practicable to vary the space angle or azimuth from which the transmission is received by the receiver, but any characteristic of signal transmission or reception, which may readily be varied at the receiver, may be em.- ployed.

As illustrated in Fig. 8, the genuine signal transmitter H has one characteristic varied in accordance with a predetermined program, and the receptivity adjustment of the receiver 18 is varied in accordance with the same program. The interference transmitter 20 has another characteristic Varied in accordance with a program which may be coordinated with the first program but which may also be an entirely different and unrelated program. For eliminating this interference, the receiver has an adjustment varied in accordance with the same program as the interference transmitter for causing the receiver to reject the interference. In the case of interference transmitted at different frequencies in accordance with a fixed program, the receiving apparatus may have a signal rejector which is 86 in Fig. 7; and with 83 set for quadrature a program,

varied to tune out. the interference; Although such a signal: rejector may take the form of an anti-resonant circuit or a. filter adjustable for eliminating different specific frequencies at each adjustment, this is not necessary, and the signal rejector (for variable-frequency interference) may merely be the tuning portion of the receiver IS, the resonant frequency of which is varied to make the receiver responsive to some frequency other than the interference frequency as explained in connection with Fig. 5.

For the sake of illustration I have represented in Fig. 8 and shall describe a specific system in which the variable characteristic of the genuine signal transmission is frequency, and the variable characteristic of the interference transmission is polarization. Thus, the genuine signal transmitter H is provided with a frequency changer 53 operated in. accordance with a predetermined designated program A, by a suitable device such as a synchronized drive 55A. Simultaneously, the receiving apparatus has its tuning varied by a frequency changer d driven by a synchronized drive 56A which is synchronized with the device 55A and thus operates in accordance with the samepredetermined program A.

As represented in Fig. 8, the genuine signal transmission and the. interference transmission are simultaneously radiated from an antenna system 15, and both transmissions are picked up by anantenna system 16. The requisite connections and intermediate apparatus between the transmitters and the transmitting antenna are represented diagrammatically by lines 89 and 90. Similarly, the connections between the receiving antenna 16 and receiver l8 are represented diagrammatically by theline 9|. 7

For varying the polarization pattern of the transmission from the interference transmitter 20, suitable apparatus such as a polarization changer T9 is provided. For one form of variable polarization pattern, the polarization changer 19 may take the form of the elements illustrated and explained in greater detail in connection with Fig. 6. The polarization changer 19 is driven by a separate synchronized drive operating in accordance with a predetermined program designated as program B.

For eliminating the. variable polarization interference transmission resulting from the interference transmitter 20, a portion of the receiving apparatus l2 determining the polarization reception characteristics of the receiver I8, shown as a rectangle and designaed as a signal rejector 83, is made variable. It is caused to vary in accordance with the same program, namely, program B, in accordance with which the synchronized drive 55B of the interference transmitter 20 is operated. For this purpose there is provided a drive 56B, operating synchronously with the drive 5513. The speed, variations in speed and direction, and other features of the program of the devices 5513 and 56B,.need bear no relation whatsoever to the corresponding features of the devices 55A and 56A. The genuine signal transmission may also be varied in accordance with the interference variation program provided the receiving apparatus is arranged to reject interference in accordance with theinterference-variation program.

For causing the receiver l8 to reject the interference from the interference transmitter 20, the polarization-determining portion of the apparatus, in this case the signal rejector 83, is so set that the recevier I8 is constantly responsivetio polarized input waves which are out of" phase with those resulting from the interference transmitter 2B. The receiver is therefore unresponsive to the polarized waves from the interference transmitter 20. Thus, the polarization-determining apparatus serves as a signal rejector. The form taken by the apparatus designated as the rejector 83 will, of course, depend upon the type of variation in the polarization pattern employed, which must correspond to that employed for the interference transmission. If the type of variation in polarization is that discussed in connection with Fig. 6, the polarization changer 83 may take the form of a phase-shifter suitably associated with other apparatus as described in connection with Fig. 6. As previously explained, however, for the arrangement of Fig. 8, since the polarized wave is to be rejected, the'polarizationadjusting mechanism in the apparatus of Fig. 8v is so set as to be 180 out of phase with the setting which would be employed for the system of Fig. 6, in which the receiver I8 is intended to be responsive to the variable polarizationpattern from the transmitter I I.

As many changes could be made in the above construction and many apparently widely different embodiments of this invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. A method of maintaining secrecy of radio signal transmission, which comprises transmitting radio signals in pulses adapted to be received by a pulse receiver, and masking such signals by transmitting interference having a lower peak intensity than the pulse signals but having an average intensity substantially as great as the peak intensity of the interference, said transmitted interference being of such strengthrelative to said signals as to require synchronism of receivers with the transmitted pulses.

2. A secret communication system comprising means at a first point for sending a series of pulses of high frequency energy from said first point to a second point remote from said first point, means for modulating said pulses in accordance with intelligence to be transmitted to said second point, means at said second point for receiving and demodulating said pulses to recover said intelligence, means for transmitting to said second point a signal interfering with said series of pulses, and means independent of the received pulses for varying the sensitivity of said receiving and demodulating means synchronously with said pulses for discriminating against said interfering signal.

3. A secret communication system comprising means for transmitting a series of intelligencemodulated pulses of a predetermined radio frequency to a remote point, means for transmitting at said radio frequency a signal interfering with said series of pulses at said remote point, means positioned at said remote point for receiving and demodulating said pulses, and means for rendering said receiver sensitive during said pulses and insensitive during intervals between said pulses whereb said receiver may discriminate said intelligence-modulated pulses from said interfering signal.

4. A secret communication system comprising means for transmitting intelligence-bearing radio Signals to a remote point, means for interrupting said signals according to a predetermined timepattern, means for transmitting to said remote point further signals interfering with said intelligence-bearin signals, said further signals being of such strength as to preclude utilization of said intelligence-bearing signals in a receiver dependent upon reception of said intelligencebearing signals for synchronization therewith, a receiver at said remote point adapted to receive said intelligence bearing signals, and means for varying the sensitivity of said receiver synchronously with said predetermined time-pattern for distinguishing sad intelligence-modulated signals from said further signals.

5. A secret communication system comprising means at a first point for transmitting to a second point recurrent fixed-duration impulses at a predetermined repetition frequency and characterized by durations of a smaller order of magnitude than the intervals between successive impulses, means for modulating said impulses in accordance with intelligence to be conveyed to an intended communicant at said second point, receiving means at said second point tuned to receive said impulses and gated at said predetermined repetition frequency in synchronism with said transmitted impulses independently of reception of said impulses, and means at said first point for transmitting a signal interfering with said fixed-duration impulses and characterized by longer transmission time than the duration of said impulses and so related to said recurrent impulses in the respects of carrier frequency, radiation pattern and of said impulses in non-synchronous impulse receivers.

RUSSELL I-I. VARIAN.

power as to prevent utilizat1on 18 REFERENCES CITED UNITED STATES PATENTS are of record in the Number Name Date 725,635 Stone Apr. 14, 1903 770,229 De Forest Sept. 13, 1904 1,309,459 Carson July 8, 1919 1,480,217 Mills Jan. 8, 1924 1,596,251 Hammond Aug. 17, 1926 1,622,033 Goldsmith Mar. 22, 1927 1,624,601 Horton Apr. 12, 1927 1,634,390 Zworykin July 5, 1927 1,667,792 Martin May 1, 1928 1,811,102 Adams et al June 23, 1931 1,931,631 Runge Oct. 27, 1933 2,034,775 Roosenstein Mar. 24, 1936 2,041,245 Haifcke May 19, 1936 2,072,962 Plebanski Mar. 9, 1937 2,083,653 Kasemann June 15, 1937 2,092,442 Colwell Sept. 7, 1937 2,199,634 Koch May 7, 1940' 2,211,132 Kotowski et al Aug. 13, 1940 2,212,238 Kolster Aug. 20, 1940 2,262,838 Deloraine et al. Nov. 18, 1941 2,266,194 Guanell-a Dec. 16, 1941 2,266,401 Reeves Dec. 16, 1941 2,272,839 Hammond Feb. 10, 1942 2,279,031 Cockerell Apr. 7, 1942 2,357,439 Usselman Sept. 5, 1944 2,381,444 Hansell Aug. 7, 1945 FOREIGN PATENTS Number Country Date 288,714 Great Britain Apr. 12, 1928 816,345 France Apr. 26, 1937 

